1. (Field of the Invention)
The present invention relates to a magneto-optical recording system, and more particularly to a magneto-optical system which can achieve over-writing of the information.
2. (Prior Art)
FIG. 6 shows a sectional structure of an essential part of the prior art magneto-optical recording medium, e.g., a magneto-optical disc. As shown in FIG. 6, a pair of transparent substrates 1 each has recording track position detecting grooves on one side surface thereof. A perpendicular anisotropy magnetic layer 2 formed of rare earth metal and transition metal is formed by deposition on the one side surface where the grooves are formed. Then, both the substrates 1 are bonded by an adhesive material 3 with the perpendicular anisotropy magnetic layers 2 opposed to each other. Protective layers 4 are formed by deposition between the substrate 1 and the magnetic layer 2 and between the magnetic layer 2 and the adhesive layer 3.
As shown in FIG. 7 which is a schematic illustration of a magnetized condition, the perpendicular anisotropy magnetic layer 2 generates perpendicular magnetization Ms by the sum of rare earth metal spin S.sub.RE and transition metal spin S.sub.TM in the layer thickness direction or in the vertical direction.
Recording to the perpendicular anisotropy magnetic layer 2 is carried out by thermal magnetic recording such as Curie temperature recording or compensation temperature recording. In case of the Curie temperature recording, an external magnetic field is applied to a recording portion by magnetic field generating means 5 as shown in FIG. 6. Under this condition, a laser beam 6 is irradiated through a condenser lens system 7 to the magnetic layer 2 to be recorded on the back side of the substrate 1 in such a manner as to focus on the magnetic layer 2, thereby heating the focused portion to a temperature higher than the Curie temperature and inverting the direction of magnetization by the external magnetic field, thus effecting the recording. In other words, under an erased condition or an unrecorded condition, the direction of magnetization is uniform as a whole as shown in FIG. 8. In contrast, under a recorded condition, the direction of magnetization at recording portions 2W is reversed to that at the other portions as shown in FIG. 9.
Recording to the perpendicular anisotropy magnetic layer is suitable for high-density recording. The aforementioned thermal magnetic recording such as Curie temperature recording or compensation temperature recording commonly has an advantage such that an external magnetic field required for recording or erasing information is remarkably less than that in a so-called magnetic recording using a magnetic tape or a magnetic disc.
However, actually there are various problems in applying an external magnetic field to this kind of magneto-optical recording medium. In the case that the external magnetic field is applied to only a minute region of a perpendicular magnetization layer, there has been proposed a method of generating a magnetic field in a limited minute region by forming a conductor pattern by a fine patterning technique such as a photolithography technique and supplying electric current to the conductor pattern. However, in this method, there occur technical and cost problems in applying a magnetic field to an arbitrary minute region in a large area.
Furthermore, inductance of a winding in generation of a magnetic field hinders driving of the magnetic field with a high frequency. To reduce the inductance of the winding, the number of windings is required to be reduced. However, the reduction in the number of windings creates the necessity of increasing the current value so as to generate a desired magnetic field, resulting in a large driving power source and an increase in power consumption.
Additionally, the above conventonal magneto-optical recording medium cannot effect over-writing. Namely, in a recording operation, a stray field H.sub.SF is generated by magnetization in the periphery of an area where the temperature is elevated to the Curie temperature by irradiation of a laser beam on an area where the coercive force HC is decreased in the case of the compensation temperature recording and a recording permissible (reversal of magnetization) temperature (which will be hereinafter referred to as a recording temperature) is reached. As a result, particularly in erasing, the stray field acts to eliminate the external magnetic field required for erasing information. Therefore, a large external magnetic field is required for erasing. FIG. 10 shows a condition where a part a is heated to the Curie temperature or a recording temperature by irradiating a laser beam 6 to the perpendicular anisotropy magnetic layer 2. While magnetization at the part a is eliminated at the Curie temperature for example, the stray field H.sub.SF is generated at the part a by magnetization Ms in the periphery of the part a. Accordingly, when an external magnetic field is applied to the part a to record or erase information, an effective magnetic field is affected by the stray field H.sub.SF. In recording, since a recording portion has magnetization having a direction reversed to that of peripheral magnetization, an external magnetic field H.sub.exw upon recording has a direction same as that of the stray field H.sub.SF, but an external magnetic field H.sub.exe upon erasing has a direction reversed to that of the stray field H.sub.SF. Therefore, effective magnetic fields H.sub.effw and H.sub.effe upon recording and erasing, respectively, are represented by the following equations (1) and (2). EQU H.sub.effw =H.sub.SF +H.sub.exw . . . (1) EQU H.sub.effe =-H.sub.SF +H.sub.exe . . . (2)
As the effective magnetic field upon erasing is small, the external magnetic field H.sub.exe is required to be increased.
In an ideal case where a sufficient inversed magnetic domain may be obtained without the external magnetic field H.sub.exw, the effective magnetic field H.sub.effw upon recording will be obtained by the stray field H.sub.SF only. However, even in such an ideal case, it is necessary to apply an external magnetic field H.sub.exe at least exceeding the stray field H.sub.SF upon erasing. Further, in order to effect sufficient inversion of magnetization, an external magnetic field H.sub.exe about double the stray field H.sub.SF is required. Actually, about several hundreds of Oe to several kOe of the external magnetic field are required for saturating the inversed magnetic domain upon recording. Such magnitude of a magnetic field is required for the external magnetic field H.sub.exe upon erasing.
It is necessary to reduce the stray field H.sub.SF to as little as possible, so as to reduce the external magnetic field H.sub.exe upon erasing. The stray field H.sub.SF may be reduced to some extent by making the composition of the magnetic layer 2 nearly equal to a compensation composition which may reduce saturation magnetization Ms of the magnetic layer 2. However, the coercive force Hc is increased to make magnetizing difficult and also make inspection of a magneto-optical disc, for example, as prepared troublesome. This is caused by the fact that the inspection of such a disc is generally carried out by use of a VSM (vibrating sample magnetometer) capable of generating a magnetic field of about 15 kOe. Therefore, another special measuring device is required to be used, or temperature is increased to reduce the coercive force Hc for measurement, which makes the operation complicated. Further, even when the magnetic layer 2 is prepared by reducing the saturation magnetization Ms only without increasing the coercive force Hc, the recording condition is rendered unstable, and high density recording cannot be carried out.
In the magneto-optical recording, a magneto-optical head for magneto-optically recording, reproducing and erasing information to a magneto-optical recording medium, that is, a magneto-optical head portion having laser beam irradiating means, optical lens system and magnetic field generating means is of a non-contact type where the head portion is retained and scanned at a desired distance from the magneto-optical recording medium. Therefore, the magnetic field generating means is separated from the magnetic layer of the recording medium at a considerable distance. As shown in FIG. 6, when a distance d between the magnetic field generating means and the recording medium is set to 1 mm, for example, a distance D between the magnetic field generating means and the magnetic layer 2 for recording, reproducing and erasing information by the irradiation of the laser beam 6 reaches about 2.5 mm in consideration of the thickness of the substrate 1 and the adhesive layer 3. Therefore, it is necessary to provide a considerably strong magnetic field generating means 5, so as to apply a magnetic field of several hundreds of Oe to several kOe to the objective magnetic layer 2. However, designing of such a strong magnetic field generating means 5 is accompanied by technically serious problems. In the case that the magnetic field generating means 5 is constituted of an electromagnet, for example, there will be generated a problem of power consumption or heat generation. When using a permanent magnet for the magnetic field generating means 5, it is difficult to accelerate a switching cycle of recording, reproducing and erasing, that is, a suitable inversion speed of magnetic field. When recording in a weak applied magnetic field C/N (carrier noise ratio) is low, and when erasing in a weak applied magnetic field, the previously recorded information cannot be fully erased. Therefore, when re-recording any information on the previously recorded area, that is, over-writing any information, information error is increased.
In Japanese patent Laid-Open Publication No. 59-60746, there is disclosed a magneto-optical recording medium including two magnetic layers. However, this recording medium does not have a bias magnetic field which is switched according to the present invention, which will be hereinafter described.